Microbial Genetics

Overview

The human-genetics lesson treated the genome that a molecular laboratory shares with its patients — diploid, chromosomal, and inherited in predictable patterns. Much of clinical molecular testing, however, is aimed not at the human host but at the microbes that infect it. The earlier lesson on chromosome and extrachromosomal structure introduced the idea that DNA lives in different physical forms and at different copy numbers; this lesson applies that idea to bacteria, viruses, and fungi. The goal is to understand how microbial genomes are organized, how they change, and why those properties dictate what a molecular assay can target. The recurring lesson is that microbial targets differ from human targets in copy number, conservation, and variability, and that those three properties drive the choice of what to detect.

Bacterial genome organization

A typical bacterium carries its essential genes on a single, circular chromosome that floats in the cytoplasm rather than being enclosed in a nucleus ¹. The chromosome is compact and gene-dense: there is relatively little of the noncoding spacing that characterizes the human genome, and related genes are frequently grouped together. A common arrangement is the operon, in which several genes that contribute to one task are transcribed together from a single promoter as one messenger RNA, so they are switched on and off as a unit ¹. This economy of organization means that a bacterial genome packs many functions into a few million base pairs.

Beyond the chromosome, most bacteria also carry plasmids — small, circular, double-stranded DNA molecules that replicate independently of the chromosome and can be present in one to many copies per cell ¹. The structure of plasmids was introduced in the chromosome and extrachromosomal lesson; here the important point is biological. Plasmids are dispensable for basic survival but often carry genes that confer a selective advantage in a particular environment, and chief among these are antimicrobial-resistance genes ².

  Bacterial cell
  ------------------------------------
   (   single circular chromosome   )      <- essential genes, operons
        o   o      o                        <- plasmids (independent,
         (small circular DNAs)                 often resistance genes)
  ------------------------------------

Horizontal gene transfer

Bacteria do not depend solely on passing genes to their descendants (vertical inheritance). They can also acquire DNA from other cells already alive around them — horizontal (lateral) gene transfer — and this is the central reason that traits such as antimicrobial resistance spread so rapidly through microbial populations ¹. Three mechanisms accomplish it:

• Transformation — a cell takes up free DNA released into its surroundings by other (often dead) cells and incorporates it ¹.
• Transduction — a bacteriophage, while infecting bacteria, accidentally packages host DNA and carries it into the next cell it infects ¹.
• Conjugation — two cells make direct contact and transfer DNA, frequently a plasmid, from a donor to a recipient through a physical bridge ¹.

  Transformation     Transduction          Conjugation
  --------------     ------------          -----------
  free DNA           phage carries         donor --bridge--> recipient
     |               host DNA                   (plasmid transferred)
     v                  |
   uptake               v
                      injection

Because a resistance gene riding on a plasmid can move by conjugation from one species to another, resistance acquired in one population can appear in a clinically unrelated organism without any new mutation arising ². Horizontal transfer thus turns resistance into a shared, mobile resource rather than a fixed property of a lineage.

Resistance genes as molecular targets

For the molecular laboratory, the practical consequence of mobile resistance is that the presence of a specific resistance gene can be detected directly, rather than waiting for an organism to grow in the presence of a drug. Several named genes are familiar examples of this principle. In methicillin-resistant Staphylococcus aureus (MRSA), resistance is associated with the mecA gene, which alters the target of beta-lactam antibiotics ². In vancomycin-resistant enterococci (VRE), the vanA and vanB genes confer resistance to vancomycin ². These genes are named here only as conceptual illustrations of detectable resistance determinants; the specific sequences, coordinates, and assay designs are matters for later infectious-disease and resistance-testing coursework. The general point is that a gene, once it is a defined and stable sequence, becomes a candidate target for a nucleic-acid test.

Conserved targets for detection and identification

Not every useful microbial target is a resistance gene. To identify an organism rather than characterize its resistance, a test needs a sequence that is present in the organisms of interest and reliable enough to serve as a fingerprint. The most widely used such targets are conserved genes — sequences that change very slowly over evolutionary time because their products are essential. In bacteria, the gene encoding the 16S ribosomal RNA is the classic example: it is present in essentially all bacteria and contains regions so conserved that a single test can be aimed at many species at once, interleaved with variable regions that differ enough to tell species apart ². For fungi, the analogous workhorse is the internal transcribed spacer (ITS) region of the ribosomal RNA gene cluster, used for fungal identification on the same logic of conserved flanks around a variable core ².

The biology behind this choice is general: ribosomal RNA genes are essential, are present in every cell, and often occur in multiple copies, which makes them both universal and abundant — two properties that ease detection ³. Conservation is therefore not merely an evolutionary curiosity; it is the feature that lets one assay serve broadly.

Viral genome diversity

Viruses depart from the single-circular-chromosome plan entirely. A viral genome may be made of DNA or RNA; it may be single-stranded or double-stranded; and it may be a single molecule or segmented into several pieces ¹. This diversity, introduced briefly in the extrachromosomal lesson, is the reason no single extraction or detection chemistry fits every virus — the method must match the genome it targets.

RNA viruses add a further complication. The enzymes that copy RNA genomes lack the proofreading that DNA polymerases provide, so RNA viruses mutate at a high rate ¹. A single infected host therefore carries not one uniform virus but a swarm of closely related variants — a quasispecies. This variability has direct laboratory consequences: it underlies the classification of viruses into genotypes, and it is the source of the resistance mutations that emerge under drug pressure. The application of these ideas — measuring how much virus is present (viral load), determining a virus’s genotype, and detecting resistance mutations — is taken up as later topics in infectious-disease testing; here the point is only why viral targets are so variable.

Why microbial targets differ in the laboratory

Pulling the threads together, microbial targets differ from the human targets of earlier lessons along three axes, and each axis shapes assay design:

  Property        Human target            Microbial considerations
  -------------   ---------------------   -----------------------------------
  Copy number     2 (diploid nuclear)     1 chromosome but multi-copy rRNA
                                          genes; plasmids 1-to-many per cell
  Conservation    fixed reference         conserved genes (16S, ITS) enable
                                          broad identification
  Variability     low (germline)          high in RNA viruses (quasispecies);
                                          mobile resistance genes

A target’s copy number sets how much template a specimen supplies, just as it did for nuclear versus mitochondrial DNA; a multi-copy ribosomal gene is easier to detect than a single-copy one ². A target’s conservation determines whether one assay can recognize many organisms or must be narrowly specific. A target’s variability determines whether a fixed probe will keep working or must be designed around regions that drift. Choosing what to detect — a resistance gene, a conserved identification gene, or a variable region for genotyping — is therefore a judgment about these three properties together. The amplification, detection, and genotyping methods that act on these targets are developed in later modules; this lesson closes the foundational genetics sequence by establishing what makes a microbial target worth choosing in the first place.